The present invention relates, in general, to an arrangement (method and apparatus) for drawing up a single crystal using a double crucible and, in particular, to crucible recovery.
The single crystal referred to herein includes Groups III-V semiconductor single crystals of GaAs, InP, GaP, etc.; Groups II-VI single crystals of ZnSe, ZnS; CdTe, etc.; and semiconductor single crystals and oxide crystals of Si, Ge, etc.
The single crystal drawing-up process includes the Czochralski process (CZ process), a liquid capsule process (LEC process), etc.
The process of drawing-up a single crystal using double crucibles (hereinafter, referred to as a double-crucible process) is used for uniformly doping impurities. In the double-crucible process, an inner crucible is put in an outer crucible so that there is communication between the inner crucible and the outer crucible through a thin through hole in the inner crucible. A single crystal is drawn up out of molten raw material in the inner crucible.
Generally, a coefficient of impurity segregation in molten raw material is not 1, and, therefore, the concentration of impurity in the molten material varies as a single crystal grows therein. The double-crucible process is effective in preventing this variation. If the impurity concentration in the inner crucible is set to be 1/k of that in the outer crucible, where k represents the coefficient of impurity segregation, it is possible to greatly reduce the variation in impurity concentration.
It is important to recover a crucible for the purpose of reuse because a crucible is expensive. PBN crucibles are generally recovered and reused, while quartz crucibles are frequently used and then discarded.
In order to recover a crucible, it is necessary to remove solid matter raw material remaining in the crucible. Solid raw material per se may be easily removed. When a liquid sealing agent such as B.sub.2 O.sub.3 or the like is used, however, it becomes difficult to remove the solid matter because the sealing agent also solidified.
Even in a single-crucible process, it is difficult to remove solidified matter that is composed both of solidified raw material and of solidified sealing agent. To remove the solidified matter, a suitable agent is poured into the crucible to dissolve the surface of the solidified matter so as to form a gap between the crucible and the solid matter. The crucible is then bent so as to separate the solidified matter therefrom.
It is more difficult to remove solidified matter in a double-crucible process because the inner crucible is fixed in solidified raw material and solidified sealing agent. The remaining solidified matter adheres to both the inner and outer surfaces of the double crucible. A method has been proposed for recovering the inner crucible wherein the inner crucible is removed from the outer crucible and hung up after the single crystal has finished growing and while the raw material is still in a molten state.
An inner crucible hanging-up apparatus such as that described above has been disclosed, for example, in European Patent Unexamined Publication No. 124,938 (laid open Nov. 14, 1984). FIG. 8 shows the conventional baffle plate type double-crucible hanging-up apparatus as disclosed in that publication. In the drawing, inner crucible 31 is connected to up/down driving rod 27 through hangers 25, up/down driving rod 27 being a separately provided up/down movable member for making it possible to cause inner crucible 31 to move vertically.
After growth of the single crystal has been completed, the single crystal is drawn up and the output of the heater is gradually reduced. At this time, molten raw material and molten sealing agent remain in the inner and outer crucibles. While the raw material and the sealing agent are in their molten states, up/down driving rod 27 is pulled up thereby taking inner crucible 31 out of the molten materials which molten materials include the molten raw material and the molten sealing agent. The temperature in the vessel accommodating the crucibles is gradually decreased with inner crucible 31 maintained in the state separated from the molten materials, so that the molten materials are solidified in the outer crucible. Thus, it is easy to recover the inner crucible because the molten materials have been excluded therefrom.
In this prior art it is not intended to recover the inner crucible but to move the inner crucible down corresponding to the descent of the surface of the molten materials as detected by sensor 29. It is easy to keep the impurity concentration in the molten materials constant because the level of molten materials in the inner crucible can be kept constant.
In the arrangement described above, it may be possible to keep the inner crucible in the state drawn up out of the raw material. This is no more than presumption on the part of the Applicant, however, since the foregoing European Patent Unexamined Publication does not describe this matter.
It is difficult to add such an up/down movement device to a single crystal drawing-up apparatus which device moves only the inner crucible up and down. The arrangement of a single drawing-up apparatus becomes complicated if an inner crucible up/down moving device is added because the whole apparatus is accommodated in a pressure-resistant vessel which vessel can be evacuated and pressurized.
In the case of a single crystal drawing-up apparatus for preparing a single crystal of a compound semiconductor, it is particularly difficult to add an inner crucible up/down moving device as described above because the entire apparatus is accommodated in a high-temperature and high-pressure vessel. There is an additional disadvantage in that it is impossible to rotate the inner crucible if it is connected to inner crucible hanging wires. Generally in a single crystal drawing-up apparatus, the single crystal and crucible are rotated independently from each other, and the diameter of the single crystal is controlled by controlling the respective rotational speed. Therefore, it is impossible to expect stable growth of the single crystal if the inner crucible cannot be rotated.